I have a script that writes to stdout like this:
# Send.py
import time
while(1):
print "hello"
time.sleep(0.1)
Now I have another script that needs to read it as such:
# Listen.py
while(1) :
print "reading.."
data = sys.stdin.read()
print data
python Send.py | python listen.py
Unfortunately, this just stops on reading and nothing is every printed. Why is that?
read() method will read until EOF, but the first program never end; read() will not return.
You need to read line by line.
Replace the following line:
data = sys.stdin.read()
with:
data = sys.stdin.readline()
In addition to that, readline() will return the read line with newline, and the print statement append new line after the string resulting empty lines inbetween. To prevent that, use sys.stdout.write:
data = sys.stdin.readline()
sys.stdout.write(data)
UPDATE
The shell will buffer output. To prevent it, flush the output:
import sys
import time
while 1:
print "hello"
sys.stdout.flush()
time.sleep(0.1)
Alternatively, you can use -u option to make stdout, stderr unbuffered.
python -u send.py | python listen.py
readline is still blocking until it will receive a \n because the output is finished the readline will continue blocking for ever or until a \n appears (well, that will be never...).
readline won't block forever, just BLOCKSIZE / len("hello\n") * .1 seconds, or about 60 seconds.print statement write the string with a newline (\n)This is because the program has not terminated yet and the read() is waiting for more io.
The pipe is like a blocking socket.
you may want to lookup python queues and make use of get_nowait()
Here is a answer that may help you: Non-blocking read on a subprocess.PIPE in python
Try to look up blocking and non-blocking io in python too.
